In simulation applications the temporal reproduction of an image is very important. Actually, the standard is set by CRT monitors or projectors, where the image is formed on the phosphor and decays very rapidly after it was written. Because of this rapid decay, there is a very limited ‘hold’ effect, where the image stays at the same place for a certain time. In projectors using transmissive or reflective light valves, like an LCD, LCOS or DLP, the image is typically present for a longer time on the light modulating means resulting in a larger ‘hold’ effect. This causes some blurring of moving images when the eye is tracking a moving object.
Projection systems wherein image generation is based on light valves are known from prior art. A typical projection system based on a single light modulating means using light valves for generating image information typically comprises an illumination module with a lamp which generates white light, a rotating color wheel which is placed in the optical path, which color wheel consist of red, green, blue and in some cases also white filter segments and a light modulating means, e.g. an LCD, LCOS or DLP device, which is illuminated by the light which was filtered by the color wheel. As a consequence, the illumination will be a temporal sequence of red, green, blue and optionally white light, in this or in another sequence. The speed of the color wheel is synchronized to the refresh rate of the image, so that every new image goes through equivalent sequences of color filter segments on the color wheel. The modulation of the light modulating means is adapted to the sequence of colors that illuminate the device, if the color of the light is for instance red, then the information for the red image is presented to the light modulating means. This ordering of information in line with the amount, order and width of the different color intervals is called “a sequence”. Such a sequence may e.g. be calculated by a tool provided by Texas Instruments, the manufacturer of DLP devices, and downloaded into the projector. Sometimes a sequence consists of a multiple times a red, green, blue and optionally white illumination interval. This means that the red, green, blue and white image information is split over more than one subframe, which helps to improve a well-known artifact called color break-up. This system can be provided by a color wheel with N times a red, green, blue and optionally white segment, or more practically by a color wheel with only once the red, green, blue and optionally white segment but rotating at N times the speed, or in other words the rotation frequency of the color wheel is in that case N times the refresh frequency of the image. In this case the term “2× color wheel” or “3× color wheel” or in general “N× color wheel” is often used. FIG. 1a shows an example of illumination timing, i.e. luminance intensity as a function of a frame time, on a light modulating means based projector with a 2× RGB color wheel. The illumination R, G and B is repeated and therefore occurring twice during the frame time of the image. FIG. 1b shows the influence of the light modulating means operation on the illumination light in one illumination interval, i.e. for a single illumination of the light modulating means with one color. The illumination on the light modulating means is denoted by the striped line 10. The light modulating means will, depending on its state, either reflect the illuminated light towards the projection lens and hence to the screen, or not reflect it towards the projection lens. As a result the light on the screen will be modulated i.e. as denoted by the full line 12.
Recently some digital light processing (DLP) projectors were demonstrated where the illumination is done by LED devices. In this case the color wheel can be omitted, and replaced by a system where the light from three types of LED devices, namely red, green and blue LEDs, is combined into one optical path and is used to illuminate the DLP device. In this case the color sequences of red, green, blue and optionally also white on the DLP device can be generated by sequentially pulsing the LEDs. For instance, for making the red illumination only the red LED is turned on, while the green and blue LED are turned off. For making white all three color LEDs can be turned on together. Just like the situation with the color wheel, also here the LED pulsing has to be synchronized to the image information on the DLP device.
It is a disadvantage of the above-described systems that the quality for displaying and projecting moving images still is not optimum, resulting in e.g. some blurring of the images.